In your device manager, are you saying that there is no video driver of any sort listed at all? - no VGA one, no unsupported video display?
If thats the case you may need to manually add a standard VGA driver first which can easily be done via "add hardware" in your control panel. After that re-boot and try installing the ATI driver again.
If thats the case you may need to manually add a standard VGA driver first which can easily be done via "add hardware" in your control panel. After that re-boot and try installing the ATI driver again.